Apparatus and method for determining the content of chemical elements in a solid sample

ABSTRACT

An apparatus is disclosed, to be used in the spectrophotometric analysis of an atomized element, which can be readily shifted from the absorption technique of analysis to the fluorescence technique. In both cases the element to be analyzed is atomized, that is reduced to its basic atomic form by subjecting it to bombardment by the positive ions of a plasma produced within an analysis cell where two electrodes are introduced one of which comprises the element to be analyzed and functions as a cathode. An electric field is applied across said electrodes for producing the plasma while the same electrodes are maintained at a very low temperature by means of a cryogenic fluid and the analysis cell is maintained at a pressure in the range from 0.1 to 50 micron Hg. Some of the apparatus components are readily interchangeable for shifting from the absorption mode of analysis to the fluorescence mode of analysis.

ilniied Mates Hordonaii et all.

APPARATTE AND METHD TFUIR DETERMINING THE QCUNTTENT @TF CHEMTQAILEILEWNTS TN A. SUILW SAMPLE Inventors: Corrndo iiiordonali; MariaAntonietta miancitiori, both of Rome, Italy Assignee: (IomitatoNazionalle per BEnergia Nucleare-nen Filed: Sept. 24, 1970 App]. No.:75,188

Foreign Application Priority Data Oct. 4, 1969 Italy ..40543-A/69 US.Cl. ..356/d5, 250/43.5 R, 250/49.5 P, 250/71 R lint. (Z1. ..G'li]l j3/30 Field oi Search ..250l43.5 R, 49.5 R, 49.5 P,

[56] Reierences Cited UNITED STATES PATENTS 2,381,414 8/1945 Wilkie..250/7l 3,217,162 11/1965 Wehner ..250/49.5 R

Primary Examiner-Anthony L. Birch Att0rneyRichards & Geier [57] ABSTRACTAn apparatus is disclosed, to be used in the spectrophotometric analysisof an atomized element, which can be readily shifted from the absorptiontechnique of analysis to the fluorescence technique. In both cases theelement to be analyzed is atomized, that is reduced to its basic atomicform by subjecting it to bombardment by the positive ions of a plasmaproduced within an analysis cell where two electrodes are introduced oneof which comprises the element to be analyzed and functions as acathode. An electric field is applied across said electrodes forproducing the plasma while the same electrodes are maintained at a verylow temperature by means of a cryogenic fluid and the analysis cell ismaintained at a pressure in the range from 0.1 to 50 micron Hg. Some ofthe apparatus components are readily interchangeable for shifting fromthe absorption mode of analysis to the fluorescence mode of analysis.

4 Ciaims, 6 Drawing Figures PATENTEB m: 8 m2 SHEET 1 OF 4 INVENTORS C.fiorclonalcwlllhBmncfwn (iidxwdlg -gdef ATTOIDGSS Fig.3

INVENTORS Bordonah'm MABc'anqffzbri diyukwu vii r PATENTEB m4 1 8 m2SHEET 3 0F 4 UyVENTpR; C, Bordonajc MMABLancLfwrz.

ATTO ILNEB5 AlPPAhliTlUfi AND METHOD EUR lDETEll lli/llllNllNG TllilECONTENT E CHEMICAL ELEMENTS TN A SOME SAMPLE The present inventionrelates to an apparatus to be used in the spectrophotometric analysisand more specifically in those analysis wherein the atomic absorptionand atomic fluorescence techniques are applied, that is those techniqueswherein an ionic bombardment is used for the direct atomization of asolid sample to be analyzed; the term atomization being here employed tosignify a process whereby from a sample the constituent atoms arereleased in a basic atomic form.

The spectrophotometric analysis through atomic absorption, as wellknown, consists of measuring the attenuation of the radiation emitted bya source at a given wavelength which attenuation is caused when theradiation is passed through a space wherein the atoms are contained of achemical element which atoms are at the temperature and pressureconditions suitable for causing the absorption of said radiation andtherefore will be called hereinafter absorbing atoms.

The spectrophotometric analysis through atomic fluorescence consists ofmeasuring the intensity of the radia tion emitted at suitableconditions, by atoms of a given chemical element when they are excitedat appropriate characteristic frequencies, which atoms will be calledhereinafter fluorescent atoms.

The apparatus usually employed for applying the above techniquescomprise the following major components:

radiation sources (adapted for the absorption technique and for thefluorescence technique respectively);

focalizing systems;

atomizing devices that is devices for producing the absorbing and/orfluorescent elements in a basically atomic form;

a measuring cell which may be also called an analytical cell;

a monochromator;

measuring systems of the intensity of the radiation con cerned.

The present invention specifically relates to devices used for producingin continuous operation the absorbing and fluorescent atoms and to themethods for using the same.

In general the means currently used for releasing the atoms from a solidsample to be analyzed (particularly in the case of absorptiontechniques) may be classified on the grounds of their operation whichmay be continuous and discontinuous; the term continuous being meant tosignify an analysis apparatus wherein the composition of atomic vaporreleased from the sample is constant throughout the operation period,which is not the case with the discontinuous operation. Thediscontinuously operating means include the following: chemical flamesproduced by the reaction of a combustible with a combustion supporter;high-temperature furnaces under vacuum, plasmas induced by radiofrequency electromagnetic fields. Continuously operating means include:some particular hollow cathodes, lasers etc.

Leaving aside the discontinuous operating systems because of theirdissimilarity from this invention, the above-mentioned continuousoperation systems suffer from the following drawbacks:

l. The composition of the atomic vapor obtained through the atomizationof the solid sample is different from the com position of the latterboth because of the selective character of the volatilization of thesample (that is the components of the sample do not volatilizesimultaneously due to the different vaporization temperature) andbecause a gaseous diffusion may take place such diffusion being favoredby the operating pressure which is rather high; which diffusion, if notkept to a minimum, would modify the composition of the atomizedsubstance along its way across the cell with respect to the samplecomposition.

2 Operating hindrances such as the long period required for attaining asteady operation of the apparatus.

3. A limited applicability as far as the operating pressure isconcerned. More specifically, when hollow cathodes are employed it ispractically impossible to operate at pressures below about microns Hg.in as much as at low pressures and with cathodes of the current sizesthe mean free path of electrons is too long to produce a sufficientionization. On the other side an effective increase of the cathodedimensions is not feasible.

With the apparatus of this invention the above drawbacks and hindrancesare overcome or minimized. In fact:

1. By means of an effective cooling of the sample, the selectivevolatilization of the different components of the sample is preventedwhich is typical of those apparatuses which operate a thermalvolatilization.

2. Very little time is consumed for carrying out an analysis which maybe reduced in the order of a few minutes.

3. The operating pressure range is very wide. A further advantage ofthis apparatus is that the amount of sample which is atomized in a timeunit can be readily and easily varied.

Still another advantage is that the specific atomization of the samplecan be made to change with time that is it can be modulated with timeaccording to a predetermined rule whereby measures can be simplyefifected with the double beam method.

Finally by this apparatus both the above-mentioned techniques ofanalysis through absorption and through fluorescence can be carried out,the simple interchange of some parts of the apparatus being required forshifting from one technique to the other; the sample being left at itsposition within the analytical cell while the analysis technique ischanged.

The above objects and advantages are achieved with the apparatus of thisinvention by producing a plasma at low pressure (in the range from about0.1 to about 50 microns Hg.) in a space wherein an appropriate plasmogengas (argon is cited as a nonlimitative example) is contained and whereintwo electrodes are placed which are substantially flat and facing eachother at a distance which can be adjusted; one of said electrodesfunctioning as a cathode and comprising the sample to be analyzed.Across said electrode an electric field is applied by known means.

The above-mentioned plasma is produced in this apparatus by saidelectric field acting on the electrons spontaneously emitted by thecathode. This action would be perfectly adequate if the pressure withinthe analysis cell were higher than about 10 microns; however at lowerpressures it is necessary and convenient to provide an auxiliary sourceof electrons and a corresponding accelerating collecting electrode alongwith a magnetic field capable of causing the electrons to move along aspiral path whereby, as well known, the collision probabilities of theelectrons and of the plasmogcn gas atoms are increased. The positiveions of the plasma so produced are used as particles for bombarding thesample to be analyzed and atomizing it. The atoms so obtained take partinto the absorption or fluorescence process. The low pressure at whichthe atomizing process occurs according to this invention minimizes thediffusion of the atoms into the gaseous phase along their path from thecathode to the anode.

With regards to the adaptability of this apparatus to either of theanalysis techniques this is obtained through the particular constructionof the apparatus. in fact this comprises two radiation sources one forthe absorption technique and another for the fluorescence technique, asingle monochromator, a single measuring system and a single analyticalcell the center of which is aligned with both of said sources of whichone only at a time is operating. The analytical cell directlycommunicates with two pairs of chambers, the chambers of one pair beingaligned with one another, with the first named source of radiation andwith the inlet slit of the monochromator while the chambers of the otherpair are aligned with one another and with the second named source ofradiation, the chambers of each pair being placed at two opposite sidesof the analytical cell and the centerlines of the two pairs being madeto cross at the center of the analytical cell. Each chamber at the freeend thereof, that is the end opposite to the analysis cell, is equippedwith an implement such as an optical window or an electron source or anelectron accelerating and capturing electrode. Two of these implements,that is those facing respectively the two radiation sources, are to beinterchanged for shifting from one technique to the other, all the restof the apparatus remaining unchanged.

This invention will be more clearly understood from the followingdescription and accompanying drawings which illustrate a preferredembodiment given by way of a nonlimitative example thereof. In thedrawings:

FIG. 1 shows schematically in plan view the apparatus of this invention;

FIG. 2 shows a vertical cross section of the analytical cell and of apair of chambers fitted for the absorption technique. This is a verticalsection of the equipment of FIG. along the axis X-X thereof;

FIG. 3 shows a cross section of the analytical cell and of a pair ofchambers fitted for the absorption technique. This is a vertical sectionof the equipment of FIG. 5 along axis YY thereof:

FIG. 4 shows a cross section of the analytical cell and of a pair ofchambers fitted for the absorption technique. This is a cross section ofthe equipment of FIG. 5 along a plane containing axes X'X and YYthereof;

FIG. 5 shows an isometric view of the assembly comprising the atomizingmeans, the analytical cell and related chambers equipped for theanalysis by the absorption technique;

FIG. 6 shows the same assembly of FIG. 5 but equipped for the analysisby the fluorescence technique.

Referring now to FIG. 1, the reference numbers 21 and 21b indicate theradiation sources which are used respectively for the absorption and forthe fluorescence technique which sources may comprise, for instance,hollow cathode lamps, discharge tubes, microwave lamps, arcs, etc. Thesources, in any case, should be capable of producing some of the characteristic frequencies of the element under examination. Reference numbers22 and 22b indicate two modulators for the radiation beams emitted bysources 21 and 21b respectively. Each modulator comprises a disk withsectors that are alternately opaque and transparent to the respectiveradiation beam. Such discs are rotated by means of motors 23 and 23b andare so located with respect to the respective beam that the latter isperiodically interrupted thereby at a frequency which depends on thenumber of sectors of the disc and on the speed of the respective motor.This type of modulator is recited as an example only of the severalmodulators which may be used for this purpose which may act either onthe radiation beam or on the source itself.

By reference numbers 24, 24' and 24b two focalizing systems areindicated one of which comprising items 24, 24' is effective forfocalizing the beam emitted by source 21, that is the source used forthe absorption technique while the other comprising items 24b and 24' iseffective for focalizing the beam emitted by'source 21b that is thesource used for the fluorescence technique. In both cases the radiationbeam is led to pass through analytical cell 1 which is adapted tofunction both for the absorption and for the fluorescence techniques. Inthe first case the image of source 21 is focalized on the inlet slit 25of a monochromator 26 which is followed by a detecting and measuringsystem 27; in the second case the area of high fluorescence within cell1 is focalized on the same slit 25.

The detecting and measuring system 27 is of any conven tional typeadapted for amplifying and detecting an alternating signal as producedby either modulator 22 or modulator 22b.

As shown by FIGS. 2, 3, 4, 5 which all relate to the arrangement foranalysis by the absorption technique, cell 1 is directly communicatingwith a first chamber 32, a second chamber 32' a third chamber 5 and afourth chamber 6. Chambers 32 and 32' are placed at opposite sides ofcell 1 and at their ends farthest from cell 1 are closed by removableoptical windows 4 and 4' respectively. Windows 4 and 4' are providedeach with an inlet 3 and 3' through which a plasmogen gas is supplied tochambers 32 and 32' respectively when said windows are fitted thereon.The inlets 3 and 3' are adjacent to optical windows 4 and 4' in order toweep off any condensation therefrom. Chambers 5 and 6 are also placed atopposite sides of cell 1 and chamber 5 is also provided with a supplyinlet 3" for the plasmogen gas. The purpose of chamber 5 is forreceiving an electron source 5b thereinto while the purpose of chamber 6is for receiving an electrode 6b as required for accelerating theelectrons flowing from chamber 5 and collect them on the same electrode.

A coil 7 embraces chamber 5 at the outside thereof which coil is fedwith direct current from a source not shown. Coil 7 may be placed aswell within chamber 5.

When operating with the fluorescent technique, optical windows 4 withthe related gas inlet 3 is placed at the end of chamber 6 and electrode6b is placed at the end of chamber 32 (see FIG. 6).

Two electrodes 9 and 10 are included within cell 1 of which the firstcomprises the sample to be analyzed. Both electrodes have the form of aflat disc and each of them is electrically, thermally and mechanicallyconnected to a respective support means 8, 8b. As an example, theconnection between the latter and the related electrode may be asillustrated by FIGS. 2 and 3 that is by means of a threaded projectionof the electrode which engages a threaded recess of the support means.

The constituent material of electrode 10 is inert with respect to thepresent process. Support means 8, 8b are identical to one another, eachof them comprising a short thin walled cylinder which fits into acorresponding receptacle 8', 8b extending from the opposite sides ofcell 1 around an axis at right angles with the plane containing the axesof said pairs of chambers. At their end farthest from cell 1, supportmeans 8, 8b are provided with a flange adapted to make a vacuum tightseal with a corresponding flange of receptacles 8, 8b. Support means 8,8!; may be made of different lengths for changing the distance betweenelectrodes 9, 10. At their ends close to cell 1 support means 8, 8b havea partition by which a space is defined wherein a refrigerating fluid iscirculated which is fed to each support body through pipes 11 anddischarged therefrom through pipes 12.

Electrodes 9 and 10 are electrically connected to a power source (notshown) such that electrode 9, which comprises the sample, be caused tofunction as a cathode. The power source may be a DC source or an ACsource. In the first case there is no problem for connecting the sourceto the electrodes. On the other hand, in the second case a feeder shouldbe inserted between the source and the electrodes which feeder shouldinclude a means for blocking the direct current component of the totalcurrent circulating through the circuit wherein the electrodes areincluded in order that a negative charge be present on the electrodewhich function as a cathode that is electrode 9. Analytical cell 1 (FIG.4) is also connected through chambers 32 and 32' and ducts 14, 14' to avacuum pump 13. As mentioned above, analytical cell 1 is adapted forfunctioning both as an absorption cell and as a fluorescence cell.

When operating with the fluorescence technique, window 4 and electrode6b are interchanged in such a way that the configuration of FIG. 6 isobtained. Then the radiation from source 21b is directed through window4 and causes the fluorescence of the atomized sample within cell 1; as aconsequence, a fluorescence radiation beam emerges from cell 1, throughwindow 4' which beam is at right angles to the beam emerging from source21b.

With regards to the structural materials, chambers 32, 32, 5 and 6 alongwith ducts l1, l2, l4 and 14' may be made of any material either heatand electricity conducting or not but otherwise compatible with theapparatus operation. Chamber 5, when surrounded by induction coil 7,should be of nonmagnetic material for avoiding any influence on the samemagnetic field by chamber walls. As a nonlimitative example, glass andplastics or metals can be used as structural materials for chambers l,32, 32', 5 and 6. The electrode support means 8,

8b may be made of any materials provided they are good heat andelectricity conductors.

Obviously, many solutions different from the above described may beresorted to both for enhancing the absorption process, such as multiplepassage arrangements, and for suppressing the need of interchangingwindow Al with electrode 6 when shifting from one technique to theother. To the latter purpose stationary supplementary windows may be forinstance provided at the free end of chamber h, while a fifth chamberabout an axis different from axis X-X may be added for mountingelectrode 6b thereon.

The operation of the above-described apparatus when operating with theabsorption technique, is as follows:

After lighting lamp 21 of the element to be determined, the radiationbeam emitted thereby is aligned and collimated on inlet slit ofmonochromator 26. By the latter a particular wavelength is selectedamong the characteristic wavelengths of said element. Modulator 22 isthen started and the intensity of the radiation emitted by source 21 ismeasured. Electrode 9 comprising the sample to by analyzed is thenfitted on support means 8. The components of the equipment of FIG. 5 arethen assembled in such a way that a vacuum tight sealing is obtained ofcell and chambers 32, 32', 5 and s. Pipes Ill and 112 are then connectedto the source of refrigerating fluid and the latter is fed to the insideof support means 8, db. Cell l is then connected to vacuum pump 13whereby the required vacuum is obtained within cell 1. Plasmogen gas isthen admitted to cell ll through inlets 3, 3' and 3" until the desiredoperating pressure is steadily maintained within cell ll. Components hb,it) which are to function as an anode and components 8, 9 which are tofunction as a cathode are connected to a voltage generator (not shown)which, depending on the sample to be analyzed, may be a DC generator ora generator of alternate current at suitable frequency. Coil 7 is thenconnected to a proper power supply (not shown). Electron source 512 isconnected to its power supply 50, while the same source and electrode 6bare connected to the same voltage generator as electrodes 9 and 110.

As a consequence of the above preparatory steps a lowpressure plasma isformed within cell l and at the same time the atomization of the samplebegins. The sample atoms released from cathode 9 move across the spacebetween electrodes 9 and i0 and along their travel intercept theradiation beam from lamp 21 which beam, at certain frequencies, isattenuated thereby; the amount of the attenuation being a function ofthe number of atoms of the element under examination which are presentin the flow of atoms through cell l, the concentration in the sample ofthe element under examination is determined by measuring the amount ofattenuation at one of the characteristic frequencies of the element. Atthis moment, the radiation intensity which reaches the measuring system2'7 is measured. From this intensity and the corresponding intensitypreviously measured when no atomization occurred the concentration inthe sample of the element under examination is determined.

When a series of samples are to be successively examined, the followingis the procedure for changing from one sample to next in the series:

cell i is cut out from pumping system 113 by means of valves not shown;

all the atomization equipment is deenergized;

atmospheric pressure is let into cell ll;

support means 8 along with cathode 9 are withdrawn from receptacle 8; afresh cathode 9 comprising the sample to be examined is substituted forthe used one; support means 3 along with cathode 9 are inserted intoreceptacle ti and the above-mentioned steps are again carried out.

The operation of the apparatus in the case of the fluorescence techniqueis as follows:

After lighting lamp 21b of the element to be measured, the radiationbeam from lamp 2llb is collimated on cell i. Modulator 22b is thenstarted. Monochromator as is then adjusted at a particularcharacteristic wavelength of the element to be measured. The image ofthe fluorescence area within cell l is focalized on inlet slit 25 of themonochromator. The atomiz' ing equipment is then energized the same asfor the absorption technique. The intensity of the radiation is thenmeasured. lFrom this measure and the corresponding measure obtainedwithout atomization the concentration in the sample of said element isdetermined.

What is claimed is:

l. An apparatus for directly determining the content of chemicalelements in a solid sample, which apparatus comprises an analyticalcell, a vacuum system connected to said cell and capable of producing ahigh vacuum therein, two main electrodes in said cell which areconnected to a direct current generator, a source of radiation capableof radiating at one of the characteristic wavelengths of the element tobe measured, a measuring system of the radiation intensity including aninlet slit; which apparatus in addition to the currently used componentsfurther comprises two pairs of elongated chambers directly communicatingwith said cell, the chambers of each pair being disposed :at oppositesides of said cell and extending outwardly therefrom; the chambers ofone pair being provided with inlet and outlet optical windows at theirends farthest from said cell and being aligned with one another, withsaid radiation source, with the center of said cell and with said slit;the chambers of the other pair being aligned with one another and theiraxis malting an angle with the axis of said one pair; an electron sourcebeing received into one chamber of said other pair and an electronaccelerating and collecting electrode being received into the otherchamber of said other pair; said apparatus being also provided with tworeceptacles extending from said cell at opposite sides thereof andaround an axis which is at right angles with the plane containing theaxes of said two pairs of chambers; in which receptacles said mainelectrodes are received, which consist of two flat discs of which onecomprises the sample to be analyzed; said chambers being provided withinlet means for supplying a plasmogen gas thereinto and into said cell;said chambers being also connected to a vacuum system capable ofproducing a vacuum down to 0.1 microns l-lg. within the vacuum tightspace defined by the walls of said chambers and cell all together;whereby when the various components of said apparatus are energized, thegas within said cell is transformed into a plasma as a consequence ofthe electric field produced by said main electrodes and of the ionizingcollisions of said electrons with the atoms of the plasmogen gas and thesample is nonselectively atomized by the positive ions of said plasmacolliding with the sample and the amount of the element in the sample isdetermined by measuring the attenuation of said radiation due to itspassing through said analytical cell.

2. An apparatus as per claim ll wherein said chamber into which saidelectron source is received is further provided with a coil connected toa direct current source whereby the electrons emitted by said electronsource are caused to follow a spiral path.

3. An apparatus as per claim ll wherein a second radiation sourceadapted for producing a fluorescence of the atomized sample is providedin alignment with said other chamber of said other pair of chambers andwherein the inlet optical window of said one pair of chambers and saidelectron accelerating and collecting electrode are readilyinterchangeable whereby said apparatus may be changed from theabsorption mode of operation to the fluorescence mode of operation andvice versa by simply interchanging said interchangeable components.

l. An apparatus as per claim ll wherein in addition to said two pairs ofchambers a fifth chamber is provided the axis of which makes an anglewith the axes of said two pairs of chambers and lies on the same planethereof, said chamber being adapted for receiving said electronaccelerating and collecting electrode; said apparatus being alsoprovided with a second radiation source adapted for producing afluorescence of the changed from the absorption mode of operation to thefluorescence mode of operation by simply transferring said electronaccelerating and collecting electrode and fitting said additionaloptical window as above described.

2. An apparatus as per claim 1 wherein said chamber into which saidelectron source is received is further provided with a coil connected toa direct current source whereby the electrons emitted by said electronsource are caused to follow a spiral path.
 3. An apparatus as per claim1 wherein a second radiation source adapted for producing a fluorescenceof the atomized sample is provided in alignment with said other chamberof said other pair of chambers and wherein the inlet optical window ofsaid one pair of chambers and said electron accelerating and collectingelectrode are readily interchangeable whereby said apparatus may bechanged from the absorption mode of operation to the fluorescence modeof operation and vice versa by simply interchanging said interchangeablecomponents.
 4. An apparatus as per claim 1 wherein in addition to saidtwo pairs of chambers a fifth chamber is provided the axis of whichmakes an angle with the axes of said two pairs of chambers and lies onthe same plane thereof, said chamber being adapted for receiving saidelectron accelerating and collecting electrode; said apparatus beingalso provided with a second radiation source adapted for producing afluorescence of the atomized sample which second radiation source isaligned with said other chamber of said other pair of chambers and isfurther provided with an additional inlet optical window to be appliedto the last named chamber when said electron accelerating and collectingelectrode is removed therefrom and transferred to said fifth chamberwhereby the apparatus is changed from the absorption mode of operationto the fluorescence mode of operation by simply transferring saidelectron accelerating and collecting electrode and fitting saidadditional optical window as above described.